To ensure good cold-start driveability, the engine is fuelled to compensate for poor quality fuels, such as “hesitation fuel” (fuels with a high DI, or “driveability index”). Such a fuel can result in combustion that is leaner than expected given the amount of injected fuel. This can result in reduced engine speed during engine starting.
To reduce such speed drops, the engine control system can monitor engine speed. If engine speed drops too low after start-up, the fueling to the engine is increased to reduce the effects of such hesitation fuels. Such a system is described in U.S. Pat. No. 5,875,759.
However, the inventors herein have recognized several disadvantages with the above approach. In particular, for normal fuel types, of fuels with a low DI, the engine is still fueled too much since the basic calibration is for an “average” fuel. In addition, if the high DI fuels cause a decrease in engine speed (since the combustion is richer than expected), added fuel exacerbates the speed drop. Further, for hesitation (high DI) fuels, the system is reactive. I.e. the engine speed has already dropped and then action is taken. In some cases this action comes too late and the customer experiences a degraded starting or tip-in behavior.